In chemistry, an onium ion is a cation formally obtained by the protonation of mononuclear parent hydride of a pnictogen (group 15 of the periodic table), chalcogen (group 16), or halogen (group 17). The oldest-known onium ion, and the namesake for the class, is ammonium, , the protonated derivative of ammonia, . The name onium is also used for cations that would result from the substitution of hydrogen atoms in those ions by other groups, such as organic radicals, or halogens; such as tetraphenylphosphonium, . The substituent groups may be divalent or trivalent, yielding ions such as iminium and nitrilium. A simple onium ion has a charge of +1. A larger ion that has two onium ion subgroups is called a double onium ion, and has a charge of +2. A triple onium ion has a charge of +3, and so on. Compounds of an onium cation and some other anion are known as onium compounds or onium salts. Onium ions and onium compounds are inversely analogous to ions and ate complexes: Lewis bases form onium ions when the central atom gains one more bond and becomes a positive cation. Lewis acids form ions when the central atom gains one more bond and becomes a negative anion. Simple onium cations (hydrides with no substitutions) Group 13 (boron group) onium cations boronium cation, (protonated borane) further boronium cations, (protonated boranes) Group 14 (carbon group) onium cations carbonium ions (protonated hydrocarbons) have a pentavalent carbon atom with a +1 charge. alkanium cations, (protonated alkanes) methanium, (protonated methane) (Sometimes called carbonium, because it is the simplest member of that class, but that use is deprecated because of multiple definitions. Sometimes called methonium, but methonium also has multiple definitions. Abundant in outer space.) ethanium, (protonated ethane) propanium, (propane protonated on an unspecified carbon) propylium, or propan-1-ylium (propane protonated on an end carbon) propan-2-ylium (propane protonated on the middle carbon) butanium, (butane protonated on an unspecified carbon) n-butanium (n-butane protonated on an unspecified carbon) n-butylium, or n-butan-1-ylium (n-butane protonated on an end carbon) n-butan-2-ylium (n-butane protonated on a middle carbon) isobutanium (isobutane protonated on an unspecified carbon) isobutylium, or isobutan-1-ylium (isobutane protonated on an end carbon) isobutan-2-ylium (isobutane protonated on the middle carbon) octonium or octanium, (protonated octane) silanium (sometimes silonium), (protonated silane) (should not be called siliconium disilanium, (protonated disilane) further silanium cations, (protonated silanes) germonium, (protonated germane) stannonium, (protonated ) (not protonated stannane